Integrated circuits are formed upon semiconductor substrates known as wafers. A wafer holds a number of identical integrated circuits, each circuit being a die that is spaced from other identical circuits. Dies are separated from the wafer usually by a dicing operation. In the dicing operation wafers are mounted on an adhesive film and the wafer is placed in a diamond saw apparatus. The diamond saw apparatus cuts the wafer to separate the dies from each other.
After separation from the wafer individual die are ready for packaging or for otherwise connecting one die to another to form a desired system or device. To package the die, each die is removed from the adhesive substrate and placed on a lead frame. A lead frame includes a plurality of finger-like connections that extend from the periphery of the lead frame toward a center die pad. The die is mounted on the center die pad. Next, wire bonds are formed between bond pads on the die and the lead fingers of the lead frame. Thereafter, the individual die are encapsulated in plastic during a molding operation. As an alternative, the lead frame with the attached die may be mounted in a ceramic package which is then hermetically sealed. Still another alternative is to directly mount the die onto a printed circuit board that contain other die which are interconnected to form an electronic system, such as a computer.
One technique for surface mounting dies on printed circuit boards uses a solder-bump interconnection. With that technique, the bond pads of the die have solder bumps or other wettable metal terminals deposited on the bond pads of the die. The die is mounted face side down onto a printed circuit board where the solder bumps of the die are aligned to electrical contacts on the board. In a typical application, the electrical contacts are metal lands that are recessed between raised glass walls. The solder-bump forms a collapsible wall of solder. The glass dam material helps retain the solder when the solder is heated.
Such surface mountable techniques can be used without packaging the die in protective plastic or ceramic encapsulating material. As such, surface mountable techniques reduce the number of steps in fabrication of electronic systems such as computers. However, one of the drawbacks with current surface mountable techniques is that the printed circuit board receives only a single layer of die.
Accordingly, there is a need for a surface mountable technique with higher die density and multiple layers of die. In particular, there is a need for surface mounted technique and surface mounted devices which are stackable, one on top of the other. There is a need for a surface mounted technique and surface mounted apparatus where one die may be directly connected to another die using contacts on both the front and back surface of each die.